Environ. Sci. Technol. 1091, 25, 1285-1289
(35) Gartrell, M.;Craun, J.; Podrebarac, D.; Gunderson, E. J.-Assoc. Off. Anal. Chem. 1986,69,146-159. (36) Ramsey, J.; Anderson, M. Toxicol. Appl. Pharmacol. 1984, 73, 159-175. (37) Anderson, D.;Hites, R. Atmos. Environ. 1989, 23, 2063-2066.
(38) Bennett, G.;Ballee, D.; Hall, R.; Fahey, J.; Butts, W.; Osmun, J. Bull. Environ. Contam. Toxicol. 1974,11,64-69. Received for review August 6, 1990. Accepted March 4, 1991. This work was supported by Grant 87ER60530 from the US. Department of Energy.
High-Temperature Removal of Cadmium Compounds Using Solid Sorbents Mohit Uberolt and Farhang Shadman" Department of Chemical Engineering, University of Arizona, Tucson, Arizona 85721
rn Emission of cadmium compounds is a major problem
Table I Composition of Sorbents Used (wt % )
in many combustors and incinerators. In the present work, the use of solid sorbents for removal of cadmium compounds from high-temperature flue gases is investigated. The sorbents tested were silica, alumina, kaolinite, emathlite, and lime. Compounds containing aluminum oxide show high cadmium removal efficiency. In particular, bauxite has the highest rate and capacity for cadmium capture. The overall sorption process is not just physical adsorption, but rather a complex combination of adsorption and chemical reaction.
SiOz A1203 FezO, TiOz CaO MgO K,O NazO
bauxite"
kaoliniteb
emathlitec
limestoned
11.0
52.1 44.9 0.8 2.2
73.4 13.9 3.4 0.4 5.0 2.6 1.2
0.7 0.3 0.3
84.2 4.8
97.2 1.5
0.1
"Paranam bauxite from Alcoa Corp. bBurgess Pigment Co. Mid-Florida Mining Co. Pfizer Inc.
Introduction
Cadmium compounds are considered to be among the most toxic trace elements emitted into the environment during fuel combustion and waste incineration. Cadmium and cadmium-compounds are primarily used in the fabrication of corrosion-resistant metals. Cadmium is also used as a stabilizer in poly(viny1 chlorides), as electrodes in batteries and other electrochemical cells, and for numerous applications in the semiconductor industry (1). Due to this wide range of applications, cadmium is present in many municipal and industrial wastes. Cadmium is also present in coal in trace quantities (2). Consequently, emission of cadmium compounds is a problem in many waste incinerators and coal combustors. The chemical form and concentration of these compounds depend on a number of factors including feed composition and operating conditions (3). The increased use and disposal of cadmium compounds, combined with their persistence in the environment and relatively rapid uptake and accumulation in living organisms, contribute to their serious environmental hazards. The present technology is inadequate to meet the expected cadmium emission standards. Therefore, new and effective methods need to be developed and investigated for controlling the emission of cadmium and other toxic metals in combustors and incinerators. A promising technique for the removal of metal vapors from high-temperature flue gases is through the use of solid sorbents to capture and immobilize the metal compounds by a combination of adsorption and chemical reactions. The sorbent can be used in two ways: a. I t could be injected as a powder (similar to lime injection) for in situ removal of cadmium compounds. b. The cadmium-containing flue gas could be passed through a fixed or fluidized bed of sorbent. The sorbent could be used in the form of pellets, beads, or monoliths (for high dust applications). Previous studies by us and other investigators indicate that solid sorbents can be very effective in removing alkali Present address: W. R. Grace & Co., Research Division, 7379 Route 32, Columbia, MD 21044. 0013-936X/91/0925-1285$02.50/0
and lead vapors from hot flue gases (4-7). In the present work, a number of potential sorbents were screened and compared. for their effectiveness in removing cadmium compounds from hot flue gases. Details of the sorption mechanism were investigated. for the selected sorbents. E x p e r i m e n t a l Section
Materials. In the first part of this study, several model compounds and naturally available materials were evaluated as potential sorbents for removal of gaseous cadmium compounds from hot flue gases. The model compounds included silica (MCB grade 12 silica gel) and a-alumina (Du Pont Baymal colloidal alumina, technical grade). The naturally available materials included kaolinite, bauxite, emathlite, and lime. The composition of these sorbents is given in Table I. Cadmium chloride was used as the cadmium source. For the screening experiments, the sorbents were used in the form of particles, 60-80 mesh in size. For the kinetic and mechanistic study the sorbents were used in the form of thin flakes (disks). The flake geometry is easy to model and characterize by analytical techniques. All the sorbents were calcined at 900 "C for 2 h and stored under vacuum until used. All the experiments were conducted in a simulated flue gas atmosphere 80% N2, and 2% HzO. containing 15% COz, 3% 02, Equipment and Procedures. Screening Experiments. The main components of the experimental system were a Cahn recording microbalance, a quartz reactor, a movable furnace, and analyzers for determining the composition of the gaseous products. This system has been previously used for screening of sorbents for removal of lead compounds. Therefore, only the salient features of the system are described here. Details can be found in a previous publication ( 4 ) . The cadmium source was suspended by a platinum wire from the microbalance, which monitored the weight change during the experiments. A fixed bed of the sorbent particles was made by placing 100 mg of the sorbent particles on a 100-mesh stainless-steel screen in a quartz insert. All experiments in this study were performed with the source at 560 "C and the sorbent at 800 "C. This method ensured that the concentration
0 1991 Amerlcan Chemical Society
Environ. Sci. Technol., Vol. 25, No. 7, 1991
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